1. Preliminary determination of the energy minima for collision induced dissociation of water molecule losses from hydronium ion clusters with water are consistent with published values for solvation enthalpies of the proton. 2. Mass spectra showing the presence of stable clusters of water around tetraalkyl ammonium ions have been obtained. Current theories of water structure in the vicinity of ions suggests the possibility of these clusters, but direct evidence for them has not existed previously. 3. The average rate at which labile protons of angiotensin II exchange with water- has been measured by hydrogen/deuterium (H/D) exchange. We found that 12/16 labile protons exchange with water within 0.02 minutes while the remaining four exchange within 4.0 minutes. These measurements, when coupled with controlled fragmentation of the molecule, allowed its solution phase conformation to be characterized. Adding small amounts of lipid to the peptide solution before the H/D exchange showed the peptide's conformation had altered and 4/16 protons did not exchange within 2 hours. 4. Electrospray mass spectra of fresh samples of 1-40 residue beta-amyloid protein (betaA4) ranging from 250 uM to 6 uM show a mixture of monomeric and aggregated forms of the peptide. The aggregates range from dimer to tetramer and are more abundant at higher concentration. The principal monomeric ion has 5 charges resulting from the addition of 5 protons, while the principal dimer has 9 charges. After aging the samples in water baths at 37 degrees C, we observed a decrease of about 0.6 for the average charge associated with the monomer. This is interpreted as a loss in charge site availability and is associated with a conformational change in the monomer to a more compact form. This is the first report of conformational differences in betaA4 by mass spectrometry.